Seismic exploration is one of the most powerful techniques for investigating the configuration of the rock strata underlying the earth's surface. The typical end product of a seismic survey is a map, termed a "seismic depth section," which can be used to determine the thickness and orientation of the various strata underlying that portion of the earth's surface from which the survey was conducted. By correlating the seismic depth section with other geologic information, such as data concerning surface outcroppings of various strata, wellbore corings, well logs, and previous seismic sections, surprisingly detailed information concerning the outermost several kilometers of the earth's crust can be developed. The predominant use of seismic exploration is in the search for subsurface structures favorable to the existence of oil and gas reservoirs.
Seismic exploration is typically performed by initiating a series of seismic impulses at the earth's surface and monitoring at a plurality of surface locations the resulting signals reflected and refracted from the underlying formations. This reflection and refraction occurs at surfaces where there is a change in the acoustic impedance of the earth, most commonly at the interface between different strata.
In conducting seismic exploration in offshore regions, swampland and other areas covered by water, seismic impulses are provided by a source suspended within the water. The most commonly used marine seismic sources are air guns. Air guns operate by abruptly introducing into the water at a desired moment a charge of highly compressed air. The compressed air expands very rapidly, accelerating the surrounding water to establish an acoustic pulse which travels radially outward from the source. The air guns and the hydrophones used for monitoring the reflection and refraction of the acoustic pulse from the subsurface strata are towed behind a survey vessel along the path of the desired seismic survey. A control and recording system on the survey vessel operates the air guns and records the data obtained by the hydrophones.
Most modern marine air gun systems employ an array of air guns which are fired simultaneously or nearly simultaneously to yield a composite acoustic pulse having characteristics superior to those obtained from a single air gun. Use of a properly selected and positioned array of air guns yields a higher pulse amplitude, a flater and broader pulse frequency band, minimizes bubble interference, and allows the generation of directional pulses for transferring a greater fraction of the acoustic energy into the subsurface strata lying along the path of the search.
One of the greatest problems encountered in the use of an array of several air guns is obtaining proper synchronization of the firing times of the individual air guns. It is essential that the acoustic pulse released by each air gun occur at the proper instant relative to the pulses generated by the other air guns. In most types of marine seismic surveying, source firing is controlled to yield in-phase addition of the pulse from each air gun. If destructive interference of the individual acoustic pulses occurs through improperly synchronized firing, the data obtained from the survey will be inferior. Obtaining proper synchronization of the onset of the individual acoustic pulses is complicated by the fact that for all air guns there is a time lag, termed the "firing delay," from the application of the firing command signal until the acoustic pulse is initiated in the water. Accordingly, it is necessary to apply the firing command signal sufficiently in advance to account for this delay. A further complication is presented by the fact that the magnitude of the time delay varies from gun to gun. Even for a particular gun, the time delay can vary with time.
To allow proper shot synchronization it is necessary to monitor the instant of firing for each shot of each gun. This is typically accomplished by monitoring the response of a piezoelectric pressure transducer mounted within the firing solenoid of the air gun. The shipboard control and recording system receives the firing detection signal from each air gun through an umbilical connecting the air guns to the ship and updates the delay time corresponding to each air gun. The timing of the firing command signals for the air guns within each array is adjusted to account for updates to the air gun delay times, thereby maintaining optimal synchronization.
In some marine seismic surveys, periodic difficulties occur in maintaining proper firing synchronization due to degradation of the firing detection signal. It has been discovered that this is the result of interference from voltage transients superimposed on the firing detection signal. These transients are largely due to the conductor transmitting the firing detection signal receiving cross talk from the relatively high current firing command signals applied to other conductors within the umbilical. It would be desirable to eliminate or minimize this cross talk. It would be further desirable to avoid the need for relatively large gauge conductors for transmitting the firing command signal from the seismic survey vessel to the air gun array.